X
X
L
L
D
D
C
C
M
M
A
A
N
N
.
.
d
d
o
o
c
c
3
3
-
-
3
3
R
R
E
E
V
V
E
E
3-10 OPERATIONAL MODES
The XL-DC operates under one of three modes: AUTO, SURVEY and TIME. Each mode is described
below. Use Keypad or Serial Function 53 to change from one mode to another or to determine the
current mode. Refer to "KEYPAD FUNCTION 53 - OPERATIONAL MODE ENTRY/REQUEST" or
"SERIAL FUNCTION F53 - OPERATIONAL MODE ENTRY/REQUEST" in this section. The as shipped
default mode is AUTO. The default on subsequent power-up will be the mode used at the previous
power-down.
3-11 AUTO MODE
AUTO mode offers a painless solution to GPS receiver start-up and operation. Under AUTO mode, no
user input is required to properly complete an XL-DC site installation.
AUTO mode requires a minimum of 4 satellites in order to complete the installation process.
After XL-DC receiver installation or whenever it is desired to reinstall the XL-DC, select AUTO MODE to
begin the installation process.
AUTO mode consists of 3 major processes: 1) Current Position Search, 2) Current Position Averaging
and Refinement and 3) Invocation of Time Mode. Time and Frequency data and output signals are
available throughout this process, however optimal accuracy and stability are not achieved until step 2)
has been completed. With good satellite visibility, this step is complete following about twenty-four
hours of averaging.
Current Position Search: Immediately after invoking AUTO mode, the XL-DC clears the position
average and the GPS core receiver Non-Volatile Random Access Memory (NVRAM), and then begins
a satellite search. Since invocation of AUTO mode does clear the average position, the time and
frequency outputs may be disturbed. Care should be taken not to needlessly invoke the AUTO
mode.
The satellite search begins with eight satellites. After several minutes, a second set of satellites is
searched. The process continues until a satellite is acquired.
Immediately after acquisition, data lock is attempted and the satellite doppler compensation (the
change in the 1.575 GHz frequency due to the apparent satellite velocity, for terrestrial based receivers,
typically 0 to +5 kHz) is adjusted until data can be read from the satellite.
After data lock, GPS time is acquired to the 20 ms level of accuracy, and almanac data loading for the
entire constellation begins. At this time the Serial Function F53 command returns "F53 AUTO: 1 SATS"
and the first line of the display indicates "MODE: AUTO 1 SAT", giving the positioning mode of the GPS
core unit. The second line indicates "sats ##", giving the Pseudo Random Noise (PRN) number of the
satellite being tracked.
During the data loading process, additional satellites are searched. When a second satellite is acquired
and data lock is achieved, the Serial Function F53 command returns "F53 AUTO: 2 SATS" and the first
line of the display indicates "MODE: AUTO 1 SAT", giving the positioning mode of the GPS core unit.
The second line indicates "sats ##", giving the PRN number of the highest satellite being tracked. At
this time, the position of the XL-DC may be placed in the proper hemisphere, narrowing the search for
possible SV's.
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